Method for creating a grit pattern on a grindstone

ABSTRACT

The present invention concerns a method for treating or preparing a grindstone surface. The method is characterized in that a laser beam is used comprising the use of a laser beam.

TECHNICAL FIELD

The present invention discloses a method to create and optimize a gritpattern on a grindstone.

PRIOR ART

In mechanical pulping, such as ground wood (GW), stone ground wood(SGW), pressure ground wood (PGW), modified or developed versions of theabove such as super pressurized ground wood (SPGW), or chemicalpressurized ground wood (CPGW), a grindstone (or grinding stone) is usedto defibrate or liberate fibers from the wood matrix. The main principlein grinding as well as in all mechanical defibration processes is tobring the wood raw material into a cyclic oscillating stress fieldwhereby the absorbed mechanical energy breaks down the structure of thefibrous raw material.

By using different conditions during the grinding, pulps with differentproperties can be obtained. The grinding conditions such as wood type,wood feeding rate, stone composition and characteristics (amplitude,frequency), temperature, pressure, water etc., affect the yield, energyconsumption as well as obtained fiber and furnish properties. Typicalfor mechanical pulps are the high yield that is obtained, >95%, which isof course a benefit when aiming for efficient usage of raw materials.Normal pulping stone tools are usually quite large and contain aplurality of abrasive segments assembled about a cylindrical concretecore see for example U.S. Pat. No. 5,243,789. Critical aspects in thegrinding process are the pulp stone quality and particularly the surfacebeing in contact with the wood. Protruding active grits, and thephysical dimension and pattern of the grits affect not only the fiber orfurnish quality but also yield and energy consumption. In a ceramic pulpstone, the grits have typically a particle size of 0.2-0.5 mm. The gritsin a traditional new paper pulp stone is on an average of 0.6 mm apartcyclically load the fibers at frequencies in the range of 40-50 kHz.

Without bound to any specific theory, the heat, pressure and mechanicalimpact in the grinding zone affects the release of fibers and fibrils.The pulp stone usually comprise different ceramic materials such assilicon carbide, aluminum oxide, whereas the inner core is cement basedmaterial. The grit pattern and grind exposure is crucial for theperformance of the stone. Sharpening can be made with differentmechanical tools whereas condition of the surface can be made with awater jet. The water jet made at high pressure (500-2400 bar) can bemade during the production and affects therefore not the production.Besides removing impurities, this also improves the quality of the pulpand production capacity. The sharpening, on the other hand, providessharped edges in the pattern which is important for water channels andenables removal of fibers. Typical groove depth is 0.2-0.5 mm.

By sharpening the grinding stone surface, the quality of the pulp can bemaintained or improved. The dressing tool, also referred as a burr, isnormally rolled over the surface under sufficient pressure. However, thefeatures of the burr itself might affect the life time of the stone andparticularly the grinding performance and the subsequent pulp quality.This will not only affect grinding efficiency, but also the papermakingprocess and end product performance.

US 2003/0145842 A1 discloses a mechanical method or tool (burr) forsharpening or dressing a grinding stone comprising a cylindrical bodycontaining plurality of teeth.

US 2008/0250725 A1 discloses a method to use abrasive round particles ina grinding surface for making longer fibers. Abrasive proppant grit isformed by attaching proppant to a grinding stone surface using anadhesive. Salmi, A., “What should the grindstone surface look like toproduce pulp with least energy, Dissertation”, University of Helsinki,2012

One negative aspect of mechanical pulp is the high energy consumptionrequired in the pulping process, which can be as much high as 3.5MWh/metric ton. Only 1-10% of the consumed energy goes to defibrating offibers. In the publication by Salmi, A. “What should the grindstonesurface look like to produce pulp with least energy, Dissertation”,University of Helsinki, 2012, the role of the grinding stone pattern onenergy consumption is revealed. By introduction a sinusoidal curve tothe surface, it was possible to reduce grinding energy by 30%. US Patent2009/0308549 (Tuovinen, O., Device and method for defibration of wood)and Finnish Patent FI 98148 (Björkvist, T., Menetelmä ja laite puunmekaaniseksi kuiduttamiseksi, 1997) a two zone grindstone surface isproposed. The inventors called this a “kneading part” and a “looseningpart” alternating in quick sessions.

However, the solutions described above to improve and manufacture agrinding stone surface are based on mechanical or chemical treatmenttechnologies. The dressing results obtained with mechanical dressing ishighly dependent on the dressing tool sharpness, pressure, stonecomposition and other factors. The surface treatment based on the use ofproppants, on the other hand, requires also continuous control ofsurface topography and that the interface between adhesive and stoneremains strong enough. By using a mechanical tool, the surface istreated regardless it is in bad condition or not.

A purpose with the present invention is to improve the method to preparea grinding stone surface. By using a more flexible technique, thepattern can be created or fixed according to the current conditions andneeds.

The present invention is related to a process based on using a laserbeam in making a grit pattern on a surface of a grindstone. Moreover,the use of laser beam enables either etching or sintering or bothsimultaneously for preparing an optimal grinding stone surface. Incontradiction to conventional mechanical sharpening techniques, thisenables a faster and more precise solution to upgrade the pattern. Thedisclosed invention can also be equipped with an imaging system, whichhence repairs and upgrades only selected areas.

The treatment of the laser beam can be operated in a normal mode toengrave the surface and create a sharp pattern for optimizing thegrinding performance (pulp quality) or to reduce the grinding energy. Byadjusting the energy used in the laser, or the laser source, the groovescan be selectively clean from fibers or resins, and also remove part ofthe resins to get grit more protruding. The laser can also be used tosmoothening lands or to create microstructure on the lands.

Laser beam can further be used in order to fuse a coating or proppant onthe grinding core stone. According to prior art, the grits are normallyadhered with an adhesive such as a resin. However, the use of laser beamenables sintering of abrasive particles onto the stone surface byfocusing the beam on the proppant which then after fuses into the resinmatrix.

Laser technology can further be used in a smart-variable mode, suchthat, the grindstone surface first is imagined and then a specific areais repaired or treated. Such a repair might improve the grindingperformance but also facilitate cleaning, and exposure of grains,grooves, cleaning and opening of pores, improving the water flow onstone surface. It might also enable more variable pattern optimizationon the surface, which might be optimal due to seasonal effects etc.

The laser technology can be combined with existing technologies, thusoffering both mechanical patterning with e.g. laser sharpening ofgrooves and smoothening of the lands.

The laser technology can also be used to sharpen the mechanical toolsand to provide maintenance of the tools during the grindstonepreparation and maintenance.

A benefit with the present invention is that laser beam method moreefficiently makes and optimizes the grit pattern on the grindstone inorder to reduce refining energy and to better tailor-made the fibers andpulp properties.

1. A method for treating or preparing a grindstone surface, comprisingtreating or preparing the grindstone surface with a laser beam.
 2. Themethod according to claim 1, wherein the laser beam engraves a gritpattern on the grindstone surface.
 3. The method according to claim 2,wherein the laser beam creates a sharp pattern for optimizing grindingperformance.
 4. The method according to claim 2, wherein the laser beamreduces grinding energy.
 5. The method according to claim 1, wherein themethod further comprises the step of adjusting the energy of the laserbeam, such that, selectively cleaning of grooves from fibers or resinsis enabled.
 6. The method according to claim 1, wherein the methodfurther comprises the step of adjusting the energy of the laser beam,such that, part of resins is removed to get grit more protruding.
 7. Themethod according to claim 1, wherein the laser beam smoothens lands. 8.The method according to claim 1, wherein the laser beam createsmicrostructure.
 9. The method according to claim 1, wherein the laserbeam fuses proppant on the grinding surface.
 10. The method according toclaim 9, wherein the propellant etches the grinding surface.
 11. Themethod according to claim 1, wherein the laser beam sinters the surface.12. The method according to claim 11, wherein the sintering creates apattern with different functionalities.